1. Field of the Invention
The present invention relates to a liquid crystal driving device that supplies control signals for AC driving to an image signal line driving circuit even in vertical blanking intervals.
2. Description of the Related Art
In general, a liquid crystal panel is configured in such a manner that image signal lines (source lines) and scanning signal lines (gate lines) are arranged in matrix form and liquid crystal cells are formed so as to correspond to the respective crossing points of the image signal lines and the scanning signal lines. The image signal lines are driven by an image signal line driving circuit and the scanning signal lines are driven by a scanning signal line driving circuit.
It is known to supply control signals to the image signal line driving circuit in vertical blanking intervals as well as in vertical scanning intervals, which is an effective measure for preventing horizontal-line-dependent unevenness in an image display on the display screen. In liquid crystal driving devices using this measure, control signals are supplied to the image signal line driving circuit in vertical blanking intervals as well as in vertical scanning intervals. The control signals are signals for AC-driving the image signal lines varied periodically at a prescribed period corresponding to the period of a horizontal sync signal. The cotrol signals include a polarity inversion signal, a data shift start pulse signal, and a latch pulse signal.
Conventional circuits that supply control signals to the image signal line driving circuit in vertical blanking intervals as well as in vertical scanning intervals are required to satisfy the following two conditions. First, in vertical blanking intervals, control signals should be supplied at a period that is the same as or close to their period in vertical scanning intervals. Second, in vertical blanking intervals, a horizontal sync signal should be supplied at a period that is the same as or close to its period in vertical scanning intervals.
If the first and second conditions are not satisfied, that is, the periods of the control signals and the horizontal sync signal in a vertical blanking interval become much different than in the preceding vertical scanning interval, portions of the control signals at the last cycle of the vertical blanking interval may cause an erroneous operation in a control in the next vertical scanning interval. Such an erroneous operation may also occur when the length of a vertical blanking interval has been varied by one horizontal period or an odd time of less than one horizontal period.